Blender 2.8 Animation: How to select keyframes from inside the UI like in 2.79
As commented, the default setup in 2.80 is to use &#8679; Shift RMB in the middle of the animation related editors to change the current frame.

If you want to change this, you can adjust the keymap settings in the User Preferences.

You can find the frame change under animation.



And the selection settings are under Dopesheet -> Dopesheet(Global)



By changing the selection settings to use &#8679; Shift RMB and the frame change to use  LMB you can continue working the way you remember.